1. Field of the Invention
The present invention relates to a dielectric ceramics and its fabrication method, and more particularly, to a low dielectric constant dielectric ceramics for a borosilicate-based low temperature fired multi-layer substrate which can be fired at a wide temperature range above and below 900° C. and exhibits a low loss electrical property.
2. Description of the Background Art
With the development in the mobile communication and information communication field, techniques for making electronic parts compact and light emerge as key factors, for which a metallization density of a substrate needs to be heightened and individual parts or size and weight of a module needs to be reduced.
Utilization of a low temperature co-fired ceramic (LTCC) technique would lead to integration of a high-density metallization substrate and various types of passive parts, so that compact and light composite parts can be fabricated.
The LTCC technique is a material and process technique where a ceramic thick film tape with a device and a circuit printed thereon is stacked and an electrode is three-dimensionally connected, which is then co-fired at a low temperature of below 950° C. to obtain an integrated ceramic multi-layer circuit structure.
Because the LTCC substrate is fired at the temperature of below 950° C., Ag can be used as an internal electrode. Thus, it is less expensive compared to a precious metal internal electrode such as Pt used for the existing ceramic multi-layer substrate and has a superior electric conductivity.
In addition, diverse passive parts such as ‘L’, ‘C’, ‘R’, or the like, are stacked as a thick film tape type and connected through the internal electrode and a via hole, whereby the ceramic thick film parts, which have been individualized as a surface mounting device (SMD), are integrated to implement one module.
The most basic one of composition constituting the LTCC is a glass composition forming a metallization substrate having a low dielectric constant and a low dielectric loss. Such a composition contains 50–90 wt % of borosilicate-based glass composition and a filler such as 10–50 wt % of Al2O3 and SiO2.
The glass is softened at a temperature of below 700° C., forms a liquid phase near 850° C. and densificated at a temperature of 850–950° C. The filler serves to constantly maintain the shape of subscriber during a firing process and increase a mechanical intensity of a fired body.
Specifically, as the glass composition, borosilicate-based glass with a small amount of alkali oxide is commonly used thanks to its low dielectric loss value.
For example, a U.S. Pat. No. 5,242,867 discloses a composition for LTCC metallization substrate. In this art, a glass composition contains 60–80% of SiO2 and 15–30% of B2O3 as primary composition and Al2O3 of below 5% and alkali are added thereto, and as a filter, alumina is used. A mixture ratio of the filter to the glass composition is 20–60%, and Na2O, K2O, Li2O, etc. are added as alkali to the glass composition. The substrate fabricated by the composition has characteristics of having a dielectric constant of 4.2–5.6 and a dielectric loss of 0.1–0.4%.
Similar compositions have been also presented, but most of them contain alkali (R2O ), causing a problem that a dielectric loss value is high, that is, above 0.1%, even though they are advantageous for glass melting.